A heterojunction type photovoltaic element, in which an amorphous silicon layer or a micro crystalline silicon layer are deposited on a single crystalline silicon substrate, is well-known. The heterojunction can have its distinguish function when an impurity is doped on an amorphous silicon layer or a micro crystalline silicon layer.
In the amorphous silicon layer or mircocrystalline silicon layer which is doped, however, defects caused by doping increase and the characteristic of the heterojunction interface is degraded. The degradation of the interface characteristic results in a lower conversion efficiency because of a recombination of carriers in the case where these silicon layers are used for a photovoltaic element.
To overcome this problem, Japanese Patent Laid-Open No. 70183/1991 (IPC:H01L 31/04) has proposed a photovoltaic element in which the heterojunction interface characteristic is improved by interposing a substantially intrinsic amorphous silicon layer between a single crystalline silicon substrate and an amorphous silicon layer for the purpose of decreasing defects at the interface.
In a conventional photovoltaic element, many uneven sections of line- or lattice-shape etc. are formed on a surface of a substrate by such processes as etching which uses resist, or anisotropic etching which employs alkaline solutions such as potassium hydroxide (KOH) or sodium hydroxide (NaOH) solutions or mechanical groove in order to improve short circuit current brought by the optical confinement effect.
FIG. 11 illustrates a structure of a photovoltaic element having the optical confinement which improves the heterojunction interface characteristic (hereinafter it is referred as an HIT structure). As shown in FIG. 11, an intrinsic amorphous silicon layer 2 is formed on an n-type. crystalline silicon substrate 1 of which front surface has many uneven sections. A p-type amorphous silicon layer 3 is formed on the intrinsic amorphous silicon layer 2. A front electrode 4 is formed on the whole region of the p-type amorphous silicon layer 3 and a comb-like collecting electrode 5 is formed on the front electrode 4. A back electrode 6 is formed on the back surface of the substrate 1.
Although the comb-like collecting electrode 5 appears to be formed on the top of the pyramid-shape protruded section in FIG. 11, the actual width of the comb-like collecting electrode 5 is no less than 100 .mu.m. To help an understanding about the notion of the comb-like collecting electrode 5, the figure describes the electrode appears to be formed only on the top of the pyramid-shape protruded section. The actual comb-like collecting electrode 5 has a width equivalent to ten to twenty protruded sections of pyramid-shape.
In the above described conventional structure of the front surface of the substrate 1, a problem may occur when the intrinsic amorphous silicon layer 2 is formed on the substrate 1 by a plasma CVD method. When an amorphous semiconductor layer such as amorphous silicon is formed by a plasma CVD method, the thickness of amorphous semiconductor layer may not be uniform in the top a, the bottom b of the uneven section on the front surface, and the plain surface between a and b. As the thickness of the amorphous semiconductor film on the top a is thick and thin on the bottom b, particularly the amorphous semiconductor film may not be sufficiently deposited at the bottom b. In FIG. 11, the intrinsic amorphous silicon layer 2 and the p-type amorphous silicon layer 3 become thin at the bottom b, and it causes a lower open circuit voltage and short circuit between the electrode and the substrate, resulting in extremely degraded output characteristic of a photovoltaic element.
This invention has an objective to provide a photovoltaic element which solves the conventional problem as described above and improve an output characteristic and yields, and a method for manufacturing the same.